The invention relates generally to a method and system for monitoring the quality of data transmission over analog transmission lines, and, in particular, to a system and method for monitoring the quality of data transmission over an optical carrier, where the data is being transmitted as wavelength-division-multiplexed (WDM) signals.
An example of an analog transmission line is an optical fiber cable used in optical telecommunications networks. Using wavelength-division-multiplexing (WDM), it is possible to send different data signals at different wavelengths, i.e., channels, at the same time, on a single optical fiber cable. When building an optical trunk line using wavelength-division-multiplexing (WDM) technology, the signals transmitted on the optical trunk line are considered to be analog signals in order to configure the network as transparently as possible.
Because of the high reliability presently required for data transmission, the ability to determine whether the signals transmitted are adequate or faulty is of great importance.
In the case of digital signals, such as SDH (Synchronous Digital Hierarchy, the CCITT optical carrier standard similar to SONETxe2x80x94Sybnchronous Optical Network) signals, there is the relatively simple option of forming a checksum for each packet or frame of transmitted data, and transmitting this checksum with the packet or frame. At a checkpoint on the receiving end, the checksum is calculated again based on the received packet or frame. The re-calculated checksum, based on the received packet or frame, is compared to the originally formed checksum which was transmitted with the data. In this way, it is possible to make a reliable judgment concerning the quality of the transmission path or the quality of the data signals transmitted.
However, in the case of analog signals it requires considerable expenditure to form a checksum and to add it as redundant information to the actual signal. In addition, for reasonable line monitoring, the optical lines need to be terminated in an optical node where monitoring is to take place. Furthermore, it is necessary that the type of signals being transmitted (e.g. ATM, SDH) be known by the optical network node, which is usually not the case. For these reasons there is no simple way of forming checksums in the case of analog signals.
Because of the problems inherent in forming an optical xe2x80x9canalogxe2x80x9d checksum, the signal-to-noise ratio is usually used to evaluate optical signals because there is a direct correlation between the bit error rate during transmission and the signal-to-noise ratio.
It is thus the object of the invention to provide a method and device which offer a simple and reliable way of evaluating the quality of signals transmitted over analog lines.
This object is met by a method according to the present invention for monitoring the quality of data transmission over analog transmission lines, in particular, optical carriers on which the data is being transmitted as WDM signals.
In the inventive method, a fractional portion of the signal on each of the channels, i.e., wavelengths, being monitored is decoupled at the starting point of a monitoring path. An analog check signal is formed from the sum of the intensities of the decoupled signals. This analog check signal is transmitted on an additional channel, i.e., on another wavelength from the wavelengths being monitored. At a monitoring point provided for this purpose, fractional portions of the signals on the wavelengths being monitored are decoupled for a second time and at least a fractional portion of the analog check signal is decoupled as well. The signals decoupled at the monitoring point are evaluated in order to determine if there has been any change of the intensities of the signals on the wavelengths being monitored between the starting point and the monitoring point.
This object of the invention is also met by a monitoring device according to the present invention for monitoring the quality of data transmission over analog transmission lines, in particular, optical carriers such as optical fiber cables, by way of which data is transmitted as WDM signals.
The monitoring device is comprised of a first decoupling device for decoupling fractional portions of the signals on the wavelengths being monitored; a generator for generating an analog check signal from the intensities of the decoupled fractional portions; a coupling device for coupling the generated analog check signal to the optical carrier; a second decoupling device for decoupling fractional portions of the signals on the wavelengths being monitored, as well as at least a fractional portion of the analog check signal from the optical carrier at a monitoring point; and an evaluation unit for evaluating the portions decoupled at the monitoring point to determine if there has been any change in the intensities of the signals on the wavelengths being monitored between the starting point and the monitoring point.
The method according to the invention and the device according to the invention provide a simple and reliable capability for monitoring, on specific transmission paths, the quality of signals transmitted in optical networks.
According to the invention, an analog check signal is tranmitted on a wavelength exclusively provided for this purpose. The analog check signal continuously represents the overall intensity of the signals on the wavelengths being monitored. In this way, the network operator at the end of the monitoring path is provided with information which can be used to determine whether there is a difference between the overall intensity of the wavelengths being monitored at the start of the monitoring path (taking into account the decoupled parts) and the overall intensity of the wavelengths being monitored at the end of the monitoring path. If a change in overall intensity on the monitoring path is detected, the network operator knows that the transmission on one of the monitored wavelengths is not faultless, and suitable measures can be introduced. Introducing such measures can of course also consist of providing an automatic correction device for cases of deviation.
In principle, the method according to the invention and the monitoring device according to the invention can be used on any analog data transmission carriers, but above all telecommunications networks with optical transmission lines are the intended field of application.
According to one advantageous way to generate the analog check signal, the analog check signal is formed such that it is equal to, or proportional to, the sum of the intensities of the fractional portions decoupled at the starting point. By monitoring to what extent the analog check signal transmitted on another wavelength is equal to, or proportional to, a sum formed at the monitoring point of the fractional portions of the signals decoupled at this point of the wavelengths being monitored, information concerning the quality of data transmission on the monitoring path can be assessed. Only if the sum is identical or proportional, can it be assumed that the signals were transmitted without fault on the monitoring path.
According to another advantageous way of generating the analog check signal, the analog check signal is the complement of the sum of the intensities of the fractional portions decoupled at the starting point in respect to a specified value. It is preferable if this specified value equals the sum of the maximum attainable intensity of the signals on the individual wavelengths being monitored. If another value is selected, then this value must exceed the sum of the maximum attainable intensities. In this case, it is only necessary in the evaluation unit to form the sum of all signals decoupled at the monitoring point, including the monitoring signal, and to monitor this sum for constancy.
After the intensity of the signals transmitted on the optical line have been reduced by the double decoupling of fractional portions, it is preferable for said signals to be amplified. This will prevent the intensity of the signals from becoming too low, particularly after passing several monitoring paths.
If there are a large number of wavelengths to be monitored, it is furthermore advantageous if the channels, i.e., wavelengths, are segregated into groups, with an analog check signal being formed for each group, and to transmit the analog check signal on a separate wavelength. When there are wavelength groups, each having its own analog check signal, each group can be associated with a generator of its own and an evaluation unit of its own. In this way, it is possible not only to obtain information concerning the quality of the transmission of all the signals transmitted on the monitoring path, but also concerning the transmission quality in respect to specific wavelengths.
If a relevant deviation is detected between the signal intensities at the starting point of the transmission path and the monitoring point, it is preferable for automatic notification to take place. A notification unit which is connected to the evaluation unit may be provided for issuing such notification.
A particularly compact embodiment of the monitoring device according to the invention is achieved for the use of monitoring the optical carriers of a telecommunications network, in that the decoupling devices and the coupling device, and possibly also the generator of the monitoring signal and the evaluation unit, are integrated into the optical nodes of the network, thus limiting the respective transmission paths.
To generate the analog check signal, the generator requires a certain amount of time. This results in the analog check signal being slightly shifted in time compared to the signals of the wavelengths being monitored. An advantageous embodiment of the monitoring device according to the invention therefore provides delay elements which, for the purpose of compensating for the processing time in the generator, delay the fractional portions of the wavelengths being monitored, at the evaluation unit, before they are used for evaluation. If the delay time is set to the time required by the generator for generating the monitoring signal, then, during evaluation, after the delay, the temporal position of the analog check signal agrees with the fractional portions of the wavelengths being monitored, which are decoupled at the monitoring point. Thus, such delay elements provide a simple way of achieving high precision in evaluation.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.